I never really liked Myrmidons, but on the other hand Mercenaries are awesome at least in FE7 and 8, the only ones I have experience with. Also, all the trainee classes, which I think are only in FE8 - not their starting classes, but the fact that those characters turn out awesome. I mean Ross, Amelia, and Ewan - they all turn out SUPER AWESOME in the end.